The fill level of a medium in a tank can be ascertained, for example, by transmitting ultrasonic, pulse signals from a sensor and detecting the pulse signals reflected on the surface of the medium by the sensor. Ultrasonic measuring devices for distance ascertainment are composed, in the usual case, of: a sensor for signal production and receipt; and a measurement transmitter for signal evaluation and data transmission. The travel time ascertained by the measurement transmitter is a characterizing measure for the traveled path of the pulse signals from the sensor to the surface of the medium. The fill level of the medium in the tank is ascertained by subtracting the one-way, traveled path of the pulse signals from the total tank height. Corresponding fill level measuring devices are available from the Endress+Hauser under the mark “Prosonic”. Such sound, or ultrasonic, sensors for fill level measurement are applied in many branches of industry, e.g. in the foods industry, water, and wastewater, management, and in the chemical industry.
For producing the ultrasonic pulses, a piezotransducer, or a piezoelectric element, is excited with an alternating voltage, in order to cause it to oscillate. The frequency of the oscillation depends only on the velocity of sound, as a constant of the material, and on the dimensions, of the piezoelectric body. Problematic, in such case is that, most often, a heavy, large-volume transformer dimensioned for the needed pulse power is required for this.
In German Offenlegungsschrift DE 1020040208951, a simple driving of a piezotransducer is disclosed, wherein no transformer is used.
In order to reduce effort and cost per measuring point, a certain number of sensors are operated and evaluated using only a single measurement transmitter. For this, a so-called scanning circuit, or scanner, is applied, which selectively produces an electrical connection between the individual sensors and the measurement transmitter. Such scanning circuits have, as a rule, a number of relays, which are so operated by the evaluating circuit of the measurement transmitter, that the switch contacts of the relay selectively connect the sensors with the evaluating circuit and the exciter switching circuit. Such a construction is shown and described in FIG. 1.
The disadvantages of such mechanical scanning circuits arise from the fact that the switch contacts, as mechanically moving components, normally can experience only a limited number of switching cycles and often present contact problems in the form of increased contact resistance at the contact surfaces of the switch contacts, which, above all, in the case of received signals in the microvolt range, leads to problems. In order to avoid these disadvantages, U.S. Pat. No. 6,051,891 A1 discloses an electronic scanning circuit. In the disclosed electronic scanning circuit, the relay contacts are replaced by a MOSFET transistors circuit. Disadvantageous in the case of this electronic scanning circuit is that the driving of the electronic scanning circuit is difficult. In such case, always two, series connected, MOSFET transistors are needed to form one electronic switch contact. This circuit construction has also the disadvantage that, for driving the two transistors, a great circuit-technical effort must be tolerated, since the two MOSFET transistors have no common, fixed reference potential for driving them. The circuit construction is, as a result, complicated—the more so, since the electronic scanning circuit with the two MOSFET transistors must be so designed, that both the exciter signal with a high electrical current and a high voltage can be transferred from the exciter switching circuit to the sensor, as well as also low returned signals received from the sensor, without losses, for processing by the evaluating circuit.